1. Field of the Invention
The field of the invention is fluidized catalytic cracking of heavy hydrocarbon feeds and use of cyclones to separate cracked product or flue gas from catalyst.
2. Description of Related Art
Catalytic cracking is the backbone of many refineries. It converts heavy feeds into lighter products by catalytically cracking large molecules into smaller molecules. Catalytic cracking operates at low pressures, without hydrogen addition, in contrast to hydrocracking, which operates at high hydrogen partial pressures. Catalytic cracking is inherently safe as it operates with very little oil actually in inventory during the cracking process.
There are two main variants of the catalytic cracking process: moving bed and the far more popular and efficient fluidized bed process.
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking is endothermic, it consumes heat. The heat for cracking is supplied at first by the hot regenerated catalyst from the regenerator. Ultimately, it is the feed which supplies the heat needed to crack the feed. Some of the feed deposits as coke on the catalyst, and the burning of this coke generates heat in the regenerator, which is recycled to the reactor in the form of hot catalyst.
Catalytic cracking has undergone progressive development since the 40s. Modern fluid catalytic cracking (FCC) units use zeolite catalysts. Zeolite-containing catalysts work best when coke on the catalyst after regeneration is less than 0.1 wt %, and preferably less than 0.05 wt %.
To regenerate FCC catalyst to this low residual carbon level and to burn CO completely to CO2 within the regenerator (to conserve heat and minimize air pollution) many FCC operators add a CO combustion promoter. U.S. Pat. Nos. 4,072,600 and 4,093,535, which are incorporated by reference, teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Most FCC's units are all riser cracking units. This is more selective than dense bed cracking. Refiners maximize riser cracking benefits by going to shorter residence times, and higher temperatures. The higher temperatures cause some thermal cracking, which if allowed to continue would eventually convert all of the feed to coke and dry gas. Shorter reactor residence times in theory would reduce thermal cracking, but the higher temperatures associated with modern units created the conditions needed to thermally crack the feed. I believed that refiners, in maximizing catalytic conversion of feed and minimizing thermal cracking of feed, resorted to conditions which achieved the desired results in the reactor, but caused other problems which could lead to unplanned shutdowns.
Modern FCC units must run at high throughput, and run for years between shutdowns, to be profitable. Much of the output of the FCC is needed in downstream processing units, and much of a refiners gasoline pool is usually derived from the FCC unit. It is important that the unit operate reliably for years, and be able to accommodate a variety of feeds. The unit must operate without exceeding local limits on pollutants or particulates. The catalyst used is somewhat expensive, and most units require several hundred tons of catalyst in inventory. Most FCC units circulate tons of catalyst per minute, the large circulation being necessary because the feed rates are large and for every ton of oil cracked roughly 5 tons of catalyst circulation is needed.
These large amounts of catalyst must be removed from cracked products lest the heavy hydrocarbon products be contaminated with catalyst and fines. Even with several stages of cyclone separation some catalyst and catalyst fines invariable remain with the cracked products. These concentrate in the heaviest product fractions, usually in the Syntower (or main FCC fractionator) bottoms, sometimes called the slurry oil because so much catalyst is present. Refiners frequently let this material sit in a tank to allow more of the entrained catalyst to drop out, producing CSO or clarified slurry oil.
The problems are as severe or worse in the regenerator. In addition to the large amounts of catalyst circulation needed to satisfy the demands of the cracking reactor, there is an additional internal catalyst circulation that must be dealt with. In most bubbling bed catalyst regenerators, an amount of catalyst equal to the entire catalyst inventory will pass through the regenerator cyclones every 15 minutes or so. Most units have several hundred tons of catalyst inventory. Any catalyst not recovered using the regenerator cyclones will remain with the regenerator flue gas, unless an electrostatic precipitator, bag house, or some sort of particulate scrubber is added at considerable cost. The amount of fines in most FCC catalyst flue gas streams is also sufficient to cause severe erosion of turbine blades if a power recovery system is installed to try to recover some of the energy in the regenerator flue gas stream.
Most refiners now use higher efficiency cyclones, which improve separation by increasing inertial forces. Most high efficiency cyclones in FCC now force incoming vapor to circle around the barrel at least 3 times, and many require 4 or 5 or more revolutions within the cyclone before flue gas can exit. At first refiners used better cyclones simply to decrease the amount of expensive catalyst lost to the atmosphere with FCC flue gas. Now, the driving force behind cyclone improvement is more likely to be reduced particulates emissions from flue gas, and to a somewhat lesser extent fines in heavy products.
While high efficiency cyclones have increased recovery of conventional FCC catalyst in the regenerator, and increased the per pass removal of catalyst fines, they have not always reduced catalyst and fines losses to the extent desired. Some refiners were forced to install electrostatic precipitators downstream of regenerators to reduce fines emissions. Some refiners had difficulty removing fines from heavy liquid product streams such as slurry oil products to meet product specifications.
The net effect of high efficiency cyclones seemed to be an decrease in loss of conventional FCC catalyst as 60-80 micron particles, coupled with an increase in the rate of production of catalyst fines. This is because of the way high efficiency cyclones work. They subject FCC catalyst to higher inertial forces, forcing catalyst to make multiple revolutions, usually about 4 to 5 revolutions, around the inside of the barrel before being removed.
Attrition of catalyst, and also wear to the cyclone, is generally agreed to be a linear function of catalyst loading in the gas and of the number of spirals in the cyclone. High efficiency cyclones, and large amounts of catalyst traffic, combine to create a harsh environment for cyclones and catalyst.
I wanted a way to retain the good catalyst recovery characteristics of high efficiency cyclones, reduce the wear on these cyclones caused by the FCC catalyst, and reduce the wear on the catalyst caused by the cyclones. Some steps had been taken on the reactor side of the FCC process to reduce the dilute phase catalyst traffic entering the reactor cyclones, by using a rough cut separator on the riser outlet. A similar approach was discharge of a riser reactor downward within a reactor vessel from an elevation high enough above the catalyst stripper to keep the cracked product vapors from stirring up the stripper catalyst bed. While these approaches, generally directed at reducing the amount of catalyst unnecessarily introduced into a dilute phase region, will help, they will do nothing to reduce catalyst entrainment into a dilute phase above a fluidized bed caused by superficial vapor velocity through the fluidized bed. This dilute phase catalyst traffic will always be present. Resort to wider diameter reactor or regenerator vessels (to reduce vapor velocities in upper portions of a regenerator) will reduce vapor velocities, but greatly increases the size and cost of these vessels, and may create additional problems. Enlarged diameter reactor vessels could easily produce coke in stagnant regions. Enlarged diameter regenerator vessels will reduce dilute phase catalyst traffic and make the regenerator more susceptible to dangerous temperature excursions from dilute phase CO combustion or afterburning. In regenerators the presence of large amounts of catalyst in the dilute phase can be beneficial from the standpoint of a heat sink.
I realized that the conventional approaches were not adequate. High efficiency cyclones consume power, and produce fines, at least they did in the regenerator. Simply adding more stages of conventional cyclones upstream or downstream of the existing cyclones would not solve the problem, some units now have third stage cyclones. Simply stuffing more stages of cyclone separation into a vessel will usually increase dilute phase catalyst traffic, for reasons discussed in more detail hereafter.
Dilute phase traffic, and fines emissions, can increase because modern FCC regenerators and reactor vessels are filled with cyclones, usually 8 to 16 of them per stage of separation. These cyclones are a large part of the capital cost of FCC units, and are absolutely essential for operation. While it may be physically possible to add more cyclones, the extra diplegs associated with these additional cyclones decreases the area available for flue gas flow from the top of the fluidized bed to the inlet horn of the cyclones. More cyclones diplegs can increase superficial vapor velocity in the dilute phase region above a fluidized bed, causing additional entrainment into the cyclones. If enough cyclone diplegs, of sufficiently large diameter, were added to, e.g., an FCC regenerator, it could increase catalyst traffic greatly, because of the higher velocity. The situation is analogous to the problem faced by ancient bridge builders trying to span a river. Putting in more stone piles to support the bridge decreased the area available for water flow, increasing water velocity, and increasing the need for stone supports.
I realized that the way to improve the operation of high efficiency cyclones, and eliminate most of their shortcomings was to rely on a low efficiency separation device in, or just upstream of the inlet horn of the high efficiency cyclones. I realized that a low pressure drop device which did a poor job of catalyst separation, letting at least an order of magnitude more catalyst escape with the flue gas than even conventional cyclone separators, provided the ideal way to eliminate most of the wear on, and caused by, high efficiency cyclones.
This allowed me to greatly decrease wear on cyclones, and to reduce catalyst attrition and abrasion caused by cyclones. While this is very beneficial, it could also increase catalyst traffic in the dilute phase, so unique equipment was developed which could be used to greatly reduce the load on cyclones and on catalyst, without increasing catalyst traffic at all in the dilute phase regions above fluidized beds.